Hydrogenation of passivated contacts

ABSTRACT

Methods of hydrogenation of passivated contacts using materials having hydrogen impurities are provided. An example method includes applying, to a passivated contact, a layer of a material, the material containing hydrogen impurities. The method further includes subsequently annealing the material and subsequently removing the material from the passivated contact.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/203,799, filed Aug. 11, 2015, the entire content ofwhich is incorporated herein by reference.

CONTRACTUAL ORIGIN

The United States Government has rights in this invention under ContractNo. DE-AC36-08GO28308 between the United States Department of Energy andAlliance for Sustainable Energy, LLC, the Manager and Operator of theNational Renewable Energy Laboratory.

BACKGROUND

Doped polycrystalline silicon (pcSi) films have been used in varioussilicon (Si) electronic devices as interlayers between active devicelayers and metal contacts, and contribute to high gain in bipolarjunction transistors by lowering the base current and the emitterresistance. In these structures, the presence of an intermediate,tunneling thickness (e.g., <10 nm), silicon oxide (SiO_(x)) layerbetween the pcSi and the single crystal silicon wafer provides wafersurface passivation without the degradation of transport. Shallowemitters are formed by diffusing dopants from the pcSi through theSiO_(x) into the wafer via post-deposition anneals. This may becarefully optimized to avoid possible detrimental side effects, such asoxide break-up, secondary phase formation, and blistering. Additionally,dopants tend to segregate along grain boundaries and pile-up at thepcSi/SiO_(x) interface. This has been shown to increase passivation bylowering carrier mobility along grain boundaries in the pcSi, and tochemically bond to dangling bonds in the SiO_(x).

When stacks made of doped pcSi formed on SiO_(x) are used in solarcells, the SiO_(x) interlayer may provide surface passivation of theunderlying wafer. Furthermore, current from the doped pcSi layer maypass through the SiO_(x) layer (e.g., via leakage/tunneling), therebyenabling low contact resistance. Since the heavily doped pcSi isseparated from the wafer by the SiO_(x), there may be no need foradditional surface passivation (as in related art Si cells, e.g. bysilicon nitride (SiN_(x))), and the metal contacts can be applieddirectly to the pcSi. Therefore, pcSi-on-SiO_(x) contacts to Si wafersmay provide a way to mitigate metallization degradation while enablingselective carrier extraction. This has resulted in very high efficiencycells that are process-temperature tolerant.

Traditionally, pcSi has been deposited using Low Pressure or AtmosphericPressure Chemical Vapor Deposition (LP-, AP-CVD) at temperatures over550° C. However, this results in double sided deposition or awrap-around problem. Single-side approaches offer the flexibility ofadditive processes, without the need for film removal.

SUMMARY

The present disclosure provides methods for hydrogenation of passivatedcontacts.

In one example a method includes applying, to a passivated contact, alayer of a material. The material may contain hydrogen impurities. Themethod also includes subsequently annealing the material, andsubsequently removing the material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate example cell structures that have an n-typepolycrystalline silicon (n/pcSi)-on-SiO_(x) passivated contact backsurface field (BSF) coupled with a thermally diffused boron (B)-emitter,and a p-type polycrystalline silicon (p/pcSi)-on-SiO_(x) passivatedcontact emitter, respectively.

FIG. 2 is a flow diagram illustrating growth and solid phasecrystallization (SPC) of a pcSi-on-SiO_(x) passivated contact.

FIGS. 3A and 3B are graphical plots illustrating secondary ion massspectrometry (SIMS) data of a pcSi-on-SiO_(x) passivated contact formedon crystalline silicon (cSi) and annealed for 30 minutes at 850° C., forB in p/pcSi and phosphorus (P) in n/pcSi, respectively.

FIG. 4 is a graphical plot illustrating decreasing active Bconcentration with crystallization time on p/pcSi-on-SiO_(x), with andwithout a silicon nitride (SiN) interlayer.

FIGS. 5A and 5B are graphical plots illustrating SIMS data of apcSi-on-SiO_(x) passivated contact formed on cSi and annealed for 30minutes at 850° C., for hydrogen and B in p/pcSi and hydrogen and P inn/pcSi, respectively.

FIGS. 6A and 6B are scanning electron microscope (SEM) imagesillustrating blistering in a p/pcSi-on-SiO_(x) film stack formed on cSi,and residual wafer surface microtips after planarization by concentratedpotassium hydroxide (KOH), respectively.

FIGS. 7A and 7B are graphical plots illustrating lifetime measurementsof nitrogen-doped Czochralski (nCz) saw damage removed (SDR) siliconwafers that have an n/pcSi-on-SiO_(x) stack grown thereon. FIG. 7C is agraphical plot illustrating lifetime measurements of preferred floatzone (pFZ) chemical-mechanical planarized (CMP) silicon wafers, having ap/pcSi on thermal SiO_(x) stack grown thereon, compared with passivatedp/pcSi on chemical SiO_(x). FIG. 7D is a graphical plot illustratinglifetime measurements of nCz textured (TXT) silicon wafers, havingp/pcSi on thermal SiO_(x) grown thereon, compared with passivated p/pcSion chemical SiO_(x).

FIG. 8 is a graphical plot illustrating contact resistance measurementsof pcSi on low temperature oxide (LTO) SiO_(x) passivated contacts withand without a-Si:H interlayers.

FIG. 9 is a set of photoluminescence (PL) images illustratingp/pcSi-on-SiO_(x) and n/pcSi-on-SiO_(x) stacks with half samples cappedwith a-Si:H, metallized with Al and subjected to a post-metallizationanneal (PMA).

FIGS. 10A and 10B are SEM images illustrating an n/pcSi passivatedcontact on LTO (thermal) SiO_(x) and on room temperature nitric acidoxidation of silicon (RTNAOS) (chemical) SiO_(x), respectively.

FIG. 11 is a flow diagram illustrating example operations forhydrogenation of passivated contacts, in accordance with one or moreaspects of the present disclosure.

DETAILED DESCRIPTION

Exemplary embodiments of the present invention provide a method forpersistent hydrogenation of passivated contacts using materials havinghydrogen impurities, such as aluminum oxide (Al₂O₃). For instance,aluminum oxide thin films can be used to passivate silicon surfaces, andcan be grown using a number of processes including Plasma-EnhancedChemical Vapor Deposition (PECVD) and/or Atomic Layer Deposition (ALD).The deposited film may be amorphous in nature. A subsequent anneal maycrystallize and densify the deposited film. Such an anneal may have theadded benefit of capping the underlying material and releasing hydrogen,as well as forming a negative fixed charge, which may be appropriate forp-type surfaces. The released hydrogen may further passivate theunderlying materials, and may also make devices more resistant to damagefrom metallization and/or radiation.

When utilizing the capping and hydrogenation techniques described hereinon passivated pcSi contacts grown on a thin tunneling oxide, furtherpassivation of interfaces and defects may occur. Photoluminescence (PL)photography and lifetime data shows an improvement in passivation beforeand after Al₂O₃ hydrogenation of pcSi as described herein.

As one example, n- and p-type pcSi films may be applied on tunnelingsilicon oxide (SiO_(x)) to form passivated contacts to Si wafers. Theresulting induced emitter and high/low (n+−n) back surface fieldjunctions of high carrier selectivity and low contact resistivity mayprovide high efficiency Si solar cells. The tunneling SiO_(x) layers(e.g., <10 nm, <2 nm, or another thickness) may be grown by any suitablemethod, such as thermal methods or chemical methods. The SiO_(x) layergrowth may be followed by a PECVD growth of p+ or n+-doped a-Si:H. Thedoped a-Si:H may be thermally crystallized into pcSi, which may resultin grain nucleation and growth, dopant diffusion into the tunnelingoxide and the Si base wafer, and/or interface restructuring. The cellprocess may improve the passivation of both oxide interfaces andtunneling transport through the oxide.

In accordance with the techniques described herein, the passivatedcontact and/or the whole cell may be coated with Al₂O₃ grown using ALD,which may then be activated at 400° C. Such additional passivation maypersist after subsequent chemical removal of the Al₂O₃. The presentdisclosure provides various details of the method and addresses aspectsof the performance of Si solar cells treated by this method, which maybe governed by the properties of the individual layers (pcSi, tunnelingoxide) and by the process history of the cell as a whole.

By carefully tailoring the cell process steps, the techniques describedherein may avoid bulk defects, secondary phases, and/or oxide breakdown,while maintaining or improving interface stability, passivation, and/orcarrier selective transport. Furthermore, the present disclosure mayprovide improved Si cell wafer surface morphology (e.g., withoutmicropyramids) and post-deposited a-Si:H capping layers to pcSi toreduce or negate the significant challenges presented by passivatedcontact metallization due to metal diffusion and damage induced bydeposition.

While described herein within the context of solar cells, one or moretechniques of the present disclosure may additionally or alternativelybe used in various other contexts. For example, using a materialcontaining hydrogen impurities may be used as a hydrogenation source invarious structures, including thin Si (both kurfless wafers as well asgrown thin Si). As another example, the techniques described herein may,in some instances, serve as a replacement for SiN_(x) hydrogenationprocesses. The techniques described herein may also be useful in theproduction of integrated circuits, thin film transistor displays, andother electronic devices.

FIGS. 1A and 1B show example cell structures created in accordance withone or more techniques of the present disclosure. FIG. 1A shows anexample solar cell structure (e.g., solar cell 2) that includes ahydrogenated passivated contact back surface field (BSF) made up ofhydrogenated n/pcSi layer 4 on SiO_(x) layer 6. The BSF of solar cell 2is coupled with thermally diffused B-emitter layer 8.

Layer 8, in the example of FIG. 1A, is a monocrystalline silicon wafer(e.g., formed using the Czochralski process) into which a dopant, suchas B, has been diffused using a thermal method or another method. Layer6 may be applied on the back of layer 8 using any suitable process(e.g., a chemical process, a thermal process, or another process).Thereafter, a layer of pcSi (e.g., n/pcSi) may be applied on layer 6.For instance, the layer of pcSi may be grown using PECVD. This pcSi onSiO_(x) stack may serve as a passivated contact to the solar cell (e.g.,to which metalized connections could be attached).

In accordance with the techniques described herein, a layer of materialcontaining hydrogen impurities (e.g., Al₂O₃) may be applied over thepassivated contact. This process is further described below with respectto FIG. 2. In some examples, the layer of material may be applied toboth the top and bottom of the device, thereby covering the bottomsurface of the pcSi layer and the top of the layer 8. In the example ofFIG. 1A, a layer of Al₂O₃ was applied to both the top of the device,forming layer 14, and the bottom of the device (not shown). The materiallayer or layers (and other layers) may be annealed. Annealing thematerial layer(s) may cause the hydrogen impurities of the material todiffuse into the pcSi layer and/or layer 8. That is, annealing thematerial may hydrogenate the pcSi, thereby forming layer 4 as shown inFIG. 1A.

After annealing, the material layer may be removed. In some examples,the material layer may be removed from the entire device. For instance,the material layer may be removed from the bottom of layer 4 and the topof layer 8. In other examples, the material may be left on the top oflayer 8 but removed from the bottom of layer 4. In the example of FIG.1A, for instance, layer 14 remains above layer 8 while the materiallayer has been removed from the bottom of layer 4.

As shown in the example of FIG. 1A, SiN_(x) layer 15 may be applied ontop of layer 14 to create a front contact for solar cell 2. The backcontact may be covered with metal layer 10. Similarly, a metallizationprocess may be performed on at least a portion of the front contact toform layer 12. Layers 10 and 12 may collect the charges created by solarcell 2. As shown in the example of FIG. 1A, solar cell 2 utilizes a“hybrid” architecture wherein the cell's BSF is a full-area passivatedcontact, and the front is a ˜130Ω/□ thermally diffused B emitter that ispassivated by an Al₂O₃/SiN_(x) stack (e.g., layers 14 and 15).

FIG. 1B shows another example solar cell structure (solar cell 22) thathas a hydrogenated passivated BSF made up of hydrogenated n/pcSi layer24 on SiO_(x) layer 26. The BSF of solar cell 22 is coupled with apassivated contact emitter made up of p/pcSi layer 36 on SiO_(x) layer38. Solar cell 22, as shown in the example of FIG. 1B, uses tunnelingpassivated contact structures both as an emitter and a BSF.

FIG. 2 is a flow diagram showing growth and solid phase crystallization(SPC) of a pcSi-on-SiOx passivated contact. FIG. 2 illustrates alow-temperature, single-sided PECVD route to deposit doped a-Si:H ontunneling SiO_(x), which is then annealed to form pcSi-on-SiO_(x)passivated contact stacks. The present disclosure also describes theeffect of the individual layer properties (tunneling oxide and pcSi) onthe performance of the passivated contacts, and their interaction duringthe cell process. Furthermore, the present disclosure addresses themechanisms responsible for the observed contact degradation by theirmetallization, and demonstrates ways to minimize such contactdegradation.

The example of FIG. 2 may begin with a cSi wafer (e.g., wafer 42). Asspecific examples, wafer 42 may be a Norsun (4 Ω-cm) n-Cz textured (TXT)and potassium hydroxide (KOH) saw damage removed (SDR) wafer, a Topsil(2 Ω-cm) pFZ double-side polished (CMP), an intrinsic wafer, or anothertype of wafer. In some examples, wafer 42 may be RCA cleaned. Wafer 42may be subjected to thin (˜1.5 nm) growth of SiO_(x) (e.g., layers 44Aand 44B). As one specific example, layers 44A and 44B may be grownchemically using room temperature HNO₃ (RTNAOS) or thermally (LTO) in atube furnace.

Heavily doped amorphous silicon (a-Si:H) (e.g., layers 46A and 46B) maybe deposited on both sides of the wafer to produce a symmetricstructure. As one specific example, layers 46A and 46B may be depositedusing PECVD at low temperature (<350° C.) using SiH₄, H₂, and B₂H₆ orPH₃ dopant gases. During testing, the a-Si:H was also deposited on asingle side on quartz witness slides for reference.

The a-Si:H may be thermally solid phase crystallized (SPC) into pcSi(e.g., layers 48A and 48B). As one specific example, layers 48A and 48Bmay be formed in a tube furnace with N₂ flow at 850° C. for varioustimes, then subjected to a forming gas anneal (FGA) at 450° C. Inaccordance with the techniques described herein, the resultingpassivated contacts may be cleaned and further passivated with, e.g., anAl₂O₃ film grown using ALD in a Beneq reactor as further describedbelow.

Lifetime measurements of devices created in accordance with thetechniques described herein were made using a Sinton WCT-120 Lifetimetester in Generalized (1/1) mode at high-level injection. Transportmeasurements were made using Transmission Line Method (TLM) patternswith 1 μm thick evaporated Al pads, while Hall measurements were made onquartz witness samples with In dots. Photoluminescence was performedusing an 810 nm laser diode source and Si CCD camera with a user definedexposure time. Thickness measurements used an n&k Analyzer, whilescanning electron microscope (SEM) images were acquired using a FEIQuanta 600. Secondary ion mass spectrometry (SIMS) depth profiles weremeasured using 1.5 keV ion bombardment energy from an O₂ source.

In some examples, the character of the initial tunnel SiO_(x) layer maysubstantially influence the passivation and electronic performance ofthe pcSi-on-SiO_(x) stack due to density and stoichiometry deviations.High temperature, thermal SiO_(x) is dense and close to SiO₂stoichiometry, is limited in bulk defects, and may provide excellentwafer surface chemical passivation. Subsequent hydrogen treatments(e.g., using FGA, alneal, or secondary film (such as SiN_(x)) anneals)may passivate the residual bulk and interfacial dangling bonds.Chemically-grown SiO_(x) layers may be less dense andoff-stoichiometric, and may result in inferior passivation.

Subjecting the a-Si:H-on-SiO_(x) stack to a high temperature annealingstep may densify or break up the SiO_(x) layer depending on itspre-existing condition, as well as the heating rate and peaktemperature. Such a crystallization anneal effuses hydrogen, nucleatesand grows pcSi material, and redistributes dopants.

In some examples, the passivated contacts (e.g., the pcSi on SiO_(x)stacks) may be further passivated using the persistent hydrogenationtechniques described herein. For instance, a layer of material havinghydrogen impurities (e.g., layers 50A and 50B) may be applied over oneor more of the passivated contacts. As one specific example of amaterial having hydrogen impurities, layers 50A and 50B may be Al₂O₃.

The device may thereafter be annealed, causing hydrogen from layer 50Aand layer 50B to diffuse into the respective adjacent pcSi layer. As aresult, layers 48A and 48B may become hydrogenated pcSi layers (e.g.,layers 52A and 52B). The hydrogenation of layers 52A and 52B may remaineven after removal of the material having hydrogen impurities. In theexample of FIG. 2, for instance, layers 50A and 50B may be removed fromlayers 52A and 52B, respectively, using any acceptable process. In someexamples, the material containing hydrogen impurities may not be removedfrom one or both sides. That is, in some examples, layer 50A and/orlayer 50B may not be removed.

FIGS. 3A and 3B are graphical plots illustrating SIMS data of apcSi-on-SiO_(x) passivated contact formed on cSi and annealed for 30minutes at 850° C., for B in p/pcSi and P in n/pcSi, respectively. TheSIMS data shown in FIGS. 3A and 3B were taken in a low-energy regime tominimize collisional mixing manifested by exaggerated elementaldrive-in. Both the p/pcSi and the n/pcSi crystallization time seriesshown respectively in FIGS. 3A and 3B show increased shallow emitterformation by diffusion through the SiO_(x) layer into the wafer withincreasing time. Dopant pile-up at the pcSi/SiO_(x) interface and/or theSiO_(x)/cSi interface is found for both dopant types as well. Thisso-called snowplow effect has been measured at interfaces with andwithout a SiO_(x) interlayer and even when monolayer delta dopinggrowths have been implemented. The snowplow effect may be associatedwith chemical passivation of interfacial dangling bonds by dopant atoms.

Tables I and II show sheet resistivities (ρ_(sheet)) and dopantconcentrations of both p/pcSi and n/pcSi witness films on quartz viaHall measurements, contrasted with TLM results of the same films on pFZand nCz wafers. The TLM measurements show an increase in ρ_(sheet) withanneal time. This increase may be due to increased shallow emitterformation in the wafer. The Hall measurements resulted in 1−3×10²⁰active dopants (both P and B) for pcSi films on quartz, with an increasein active P and a decrease in active B with anneal time.

TABLE I p/pcSi passivated contact TLM ρ_(sheet) measurements on varioussubstrates and dopant concentration 850° C. Quartz Active Boron nCz/LTO/pFZ/CMP/ Anneal Quartz Concentration TXT LTO Time (min) (Ω/□)(×10²⁰/cm³) (Ω/□) (Ω/□) 5 1608 2.94 1884 61 30 2621 1.50 1122 59 90 30091.20 638 53

TABLE II n/pcSi passivated contact TLM ρ_(sheet) measurements on varioussubstrates and dopant concentration Quartz Active Phosphorus 850° C.Anneal Quartz Concentration nCz/SDR/LTO Time (min) (Ω/□) (×10²⁰/cm³)(Ω/□) 5 2429 1.20 171 30 792 2.94 137 90 736 3.00 101

The deactivation of B reflected in Table I above may be explained bydiffusion into the quartz, secondary crystalline phase formation, and/orcomplexing in the bulk via amorphous clustering. As shown in the exampleof FIG. 4, deactivation may also occur when similar anneal timecomparison is performed on quartz samples with and without a SiN_(x)B-diffusion barrier. Secondary phase formation or complexing may be inthe bulk of the p/pcSi or in the pile-up region during the anneal andcrystallization. Hall measurements on quartz indicate active dopantconcentrations at least an order of magnitude lower than the totalconcentration measured by SIMS. P-type films have an exceedingly highamount of B, and once the solubility limit of B in Si is reached and asecond phase (SiB_(x)) is formed, the B diffusivity may be degraded.Since dopants will be segregated, the SiB_(x) may form at grainboundaries and interfaces.

In accordance with the techniques described herein, aftercrystallization, the passivated contact stack and/or other surfaces maybe further passivated by reintroducing atomic hydrogen using FGA- and/orALD-grown Al₂O₃ deposition and activation. In some examples, SiN_(x) orremote H plasma may produce a similar effect. Table III showspassivation data from two concrete examples using single side polished(SSP) n/pcSi-on-SiO_(x) symmetric stacks on SSP wafer pieces (Montcon-Cz, 5.6 Ω-cm). Subsequent SiN_(x)/FGA and Al₂O₃/N₂ anneal treatmentswere performed, resulting in similar implied open-circuit voltage (iVoc)values. When Al₂O₃ or another suitable material having hydrogenimpurities is deposited using ALD, this passivation may be maintainedeven after the Al₂O₃ or other material is removed (e.g., by an HF dip).Thus, the Al₂O₃ or other material may be used as a hydrogen reservoirthat passivates interfaces, grain boundaries, SiO_(x) dangling bonds,and/or the wafer/SiO_(x) interface.

FIGS. 5A and 5B are graphical plots illustrating SIMS data of apcSi-on-SiO_(x) passivated contact formed on cSi and annealed for 30minutes at 850° C., for hydrogen and B in p/pcSi and hydrogen and P inn/pcSi, respectively. It can be seen in FIGS. 5A and 5B that after HFremoval of Al₂O₃ on passivated contact stacks, hydrogen tends to trackwith the B profile especially at the pile-up location, indicatingchemical passivation of the pcSi/SiO_(x) and SiO_(x)/cSi interfaces,bulk pcSi grain boundaries, and possible additional crystallographicdefects, but does not follow the pile-up region for the P profile.

TABLE III Hydrogenation of n/pcSi-on-SiO_(x) passivated contact withSiN_(x) and Al₂O₃ (with subsequent HF removal) Crystallization SiNx AsSiNx + iVoc FGA iVoc Grown iVoc FGA iVoc Sample (mV) (mV) (mV) (mV) 1656 685 677 696 Crystallization Al2O3 + iVoc FGA iVoc Anneal iVoc HF DipiVoc (mV) (mV) (mV) (mV) 2 620 660 698 697

FIGS. 6A and 6B are scanning electron microscope (SEM) imagesillustrating blistering in a p/pcSi-on-SiO_(x) film stack formed on cSi,and residual wafer surface microtips after planarization by concentratedpotassium hydroxide (KOH), respectively. In some examples, metallizationcan degrade passivated contact performance based on deposition techniqueand/or existence of blisters, pinholes, or other metal diffusion paths.Blistering of films, as seen in FIG. 6A, can occur at either side of theSiO_(x) interface during deposition or crystallization if conditions arenot carefully optimized. A further complication in depositing dopeda-Si:H precursor films is that B-doped films may effuse hydrogen atlower temperatures than P-doped films, so extra care should beimplemented during deposition and when annealing. Surface preparation isanother factor in the ultimate performance of the passivated contact.For instance, microtip features on SDR surfaces, as shown in FIG. 6B,may coincide with lifetime degradation. Such microtip features can occurfrom incomplete KOH etching.

In some examples, in order to mitigate deleterious metallizationeffects, a thin doped a-Si:H cap may be applied on passivated contacts(e.g., after further passivation with, and removal of, Al₂O₃). FIGS. 7Aand 7B show the results of lifetime measurements performed on n/pcSifilms grown on LTO (thermal) and RTNAOS (chemical) SiO_(x) on nCz SDRwafers, respectively. FIGS. 7C and 7D compare p/pcSi grown on boththermal and chemical SiO_(x) on CMP (polished) pFZ and TXT nCz wafers,respectively. The samples (referenced to the as-grown, initiala-Si:H-on-SiO_(x) stack) are crystallized and FGA, passivated withAl₂O₃, stripped with HF, and finally capped with the intermediate a-Si:Hlayer. An increase in passivation is seen once the Al₂O₃ treatment isperformed for all samples in FIGS. 7A-7D. A slight drop HO mV) may occurwhen Al₂O₃ is removed by HF on samples employing a thermal SiO_(x) or achemical SiO_(x) sample. The a-Si:H cap heals this to some degree, but asustained degradation may be present.

FIG. 8 is a graphical plot illustrating contact resistance measurementsof pcSi on LTO SiO_(x) passivated contacts with and without a-Si:Hinterlayers. Contact resistance TLM measurements with Al pads were madeusing high/low junctions using n/pcSi passivated contacts to nCz (˜2Ω-cm) wafers and p/pcSi to pFZ (˜4 Ω-cm) wafers. Additional,appropriately doped, thin (˜5 nm) a-Si:H films were deposited todetermine changes in transport behavior. All stacks show a contactresistance of <100 mΩ/cm², which is acceptable for a full area contactin 1-sun Si solar cells. Furthermore, a-Si:H doping and film thicknesscan be tailored to improve performance.

FIG. 9 is a set of PL images illustrating p/pcSi-on-SiO_(x) andn/pcSi-on-SiO_(x) stacks with half samples capped with a-Si:H,metallized with Al and subjected to a PMA. PMAs are typically applied tomitigate e-beam, X-ray, or sputtering deposition damage. The PL imagesin FIG. 9 (normalized with the same exposure time) show thecrystallization time series samples half capped with (˜5 nm) dopeda-Si:H to determine any further degradation in lifetime due tometallization. It can be seen that the 5 minute, 850° C. p/pcSi sample(f) is dark throughout due to severe blistering, caused by incompletecrystallization coupled with Al₂O₃ application. The more crystallinep/pcSi samples (d) and (e) show more metal degradation on regionswithout a capping layer, which is amplified by PMA. Conversely, then/pcSi samples (a), (b), and (c) show initial degradation due tometallization that is somewhat healed by PMA.

FIGS. 10A and 10B are SEM images illustrating n/pcSi passivated contacton LTO (thermal) SiO_(x) and on RTNAOS (chemical) SiO_(x), respectively.Blistering and film roughness, as shown in FIGS. 10A and 10B, canaccount for the significant difference in lifetime from the resultsshown in FIGS. 7A-7D employing thermal versus chemical SiO_(x). TheAl₂O₃ passivates and hydrogenates both stacks, even with the presence ofresidual microtips and the exposed wafer surface regions wheredelamination has occurred. Upon Al₂O₃ removal, passivation maintains forthe smooth, non-blistered stack using the thermal SiO_(x) as shown inFIG. 10A. However, degradation may occur in the exposed wafer surfacesas shown in FIG. 10B, where delamination exists for the chemicalSiO_(x).

Another mechanism for poor performance is oxide break-up, which canresult in solid phase epitaxy of the pcSi extending from the wafersurface, and upon metallization, can then be perceived as direct metalcontact to the wafer. Depending on the degree of dopant segregation, thebulk of the epitaxial regions can range from highly doped regions withenhanced Auger recombination but relative immunity to metal contact, tosevere dopant segregation into grain boundaries leaving lowly dopedepitaxial columns with enhanced recombination when directly contacted tometal. Thus, the SiO_(x) interlayer serves an important role inpassivation, and continuity of the SiO_(x) interlayer, as well as thecontinuity of the pcSi layer, should be maintained.

Exemplary embodiments of the present invention provide single sidedeposited pcSi on tunneling SiO_(x) as effective passivated contacts tocrystal silicon wafers, achieving over 700 mV iVoc for both n/pcSi tonCz and p/pcSi to pFZ wafers, with contact resistance below 100 mΩ/cm².The initial SiO_(x) character largely governs the eventual performanceof the contact, where a denser, more stoichiometry oxide is desirableand is more resistant to defects such as blistering and delaminationduring deposition and in subsequent process steps. The pcSi may becrystallized from a PECVD-grown a-Si:H film. Gradually increasing depthsof shallow emitter profiles were measured via SIMS and transportmeasurements with increasing anneal time. In accordance with thetechniques described herein, hydrogenation of the passivated contactstack may be effectively achieved using Al₂O₃ thin films, which can thenbe removed, leaving passivation relatively unchanged. In some examples,metallization may produce damage and decrease passivation, both byinherent process externalities as well as by metal diffusion pathspresent in the pcSi-on-SiO_(x) stack. The techniques described hereinmay effectively mitigate such metallization effects by utilizing a thina-Si:H interlayer and/or annealing.

FIG. 11 is a flow diagram illustrating example operations forhydrogenation of passivated contacts, in accordance with one or moreaspects of the present disclosure. The example operations of FIG. 11 mayrepresent only a subset of the operations performed in hydrogenatingpassivated contacts, and various other operations may be additionally oralternatively included, as described herein.

In the example of FIG. 11, one or more passivated contacts may becreated (100). The passivated contacts may be formed using any number ofsuitable methods, including PECVD, LP-CVD, Physical Vapor Deposition(e.g., e-beam or thermal), reactive sputtering, or others. The resultingpassivated contacts may be made of various materials, such as pcSi,mixed-phase polycrystalline/amorphous silicon, pcSi alloys with carbon,oxygen and other atoms, and others. In some examples, the materials canbe doped n- or p-type by various known dopant atoms (e.g., P, B,aluminum, arsenic, and other Group III and Group V elements).

A layer of material that contains hydrogen impurities may be applied tothe passivated contact(s) (102). Examples of suitable materials thatcontain hydrogen impurities may include Al₂O₃, SiO₂, SiN_(x), a-Si:H, aswell as other material layers that incorporate hydrogen (e.g., up toseveral atomic percent) as a result of their specific growth methodusing H-containing radicals such as trimethylaluminum. The material maybe applied using any suitable method, such as PECVD, LPCVD, ALD,dip-coat, spin-coat, and others.

After applying the material that contains the hydrogen impurities, theresult may be annealed (104). Annealing may cause the hydrogen todiffuse from the applied material into the underlying passivatedcontact. The anneal may be performed with various parameters, includingdifferent temperatures, rates of temperature change, and durations. As aconcrete example, the device may be heated to 400 degrees Centigradeover a period of 20 to 60 minutes.

In some examples, the applied layer of material may be removed from thepassivated contacts (106). For example, the applied layer may bechemically removed using HF, or other suitable materials. In someexamples, the applied layer may be removed in various other suitableways, such as reactive ion etching, plasma etching, and others. Othermethods may remove the applied layer by exploiting the layer's loweradhesion to the passivated contact. However, while the material may beremoved, at least a portion of the improvements to the passivatedcontacts may remain. That is, the hydrogen diffusion resulting from theannealing may further passivate the contacts, and removal of the appliedlayer may not undo these improvements. As another example, a method mayinclude applying, to a passivated contact, a layer of a material, thematerial containing hydrogen impurities; subsequently annealing thematerial; and subsequently removing the material from the passivatedcontact. In some examples, the material may be alumina (Al₂O₃). In someexamples, the material may be applied using Plasma-Enhanced ChemicalVapor Deposition (PECVD). In some examples, the material may be appliedusing Atomic Layer Deposition (ALD). In some examples, the passivatedcontact may include a layer of polycrystalline silicon (pcSi) on siliconoxide (SiOx).

In some examples, the method may include growing the passivated contacton a silicon wafer, wherein growing the passivated contact comprisesgrowing a layer of silicon oxide (SiOx) on the silicon wafer. In someexamples, the layer of silicon oxide may have a thickness of 10 nm orless. In some examples, the layer of silicon oxide may have a thicknessof 2 nm or less. In some examples, the layer of silicon oxide may bethermally grown on the silicon wafer. In some examples, the layer ofsilicon oxide may be chemically grown on the silicon wafer.

In some examples, growing the passivated contact may include growingamorphous silicon on the layer of silicon oxide. In some examples,growing the passivated contact may include thermally crystallizing theamorphous silicon to form a layer of polycrystalline silicon (pcSi). Insome examples, the amorphous silicon may be applied usingPlasma-Enhanced Chemical Vapor Deposition (PECVD).

In some examples, removing the material may include applying at leastone of an acid or a base to the material. In some examples, the methodmay include applying a doped amorphous silicon cap on the passivatedcontact after removing the material. In some examples, the dopedamorphous silicon cap may have a thickness between about 3 nm and about7 nm. In some examples, the passivated contact may be incorporatedwithin a solar cell.

The foregoing disclosure includes a number of examples set forth merelyas illustration and these examples are not intended to be limiting.Modifications of the disclosed examples incorporating the spirit andsubstance of the described methods and/or devices may occur to personsskilled in the art. These and other examples are within the scope of thefollowing claims.

What is claimed is:
 1. A method comprising: applying, to a passivatedcontact, a layer of a material, the material containing hydrogenimpurities; subsequently annealing the layer of the material; andsubsequently removing the layer of the material from the passivatedcontact.
 2. The method of claim 1, wherein the layer of the materialcomprises alumina (Al2O3).
 3. The method of claim 1, wherein the layerof the material is applied using Plasma-Enhanced Chemical VaporDeposition (PECVD).
 4. The method of claim 1, wherein the layer of thematerial is applied using Atomic Layer Deposition (ALD).
 5. The methodof claim 1, wherein the passivated contact comprises a layer ofpolycrystalline silicon (pcSi) on silicon oxide (SiOx).
 6. The method ofclaim 1, further comprising growing the passivated contact on a siliconwafer, wherein growing the passivated contact comprises growing a layerof silicon oxide (SiOx) on the silicon wafer.
 7. The method of claim 6,wherein the layer of silicon oxide has a thickness of 10 nm or less. 8.The method of claim 7, wherein the layer of silicon oxide has athickness of 2 nm or less.
 9. The method of claim 6, wherein the layerof silicon oxide is thermally grown on the silicon wafer.
 10. The methodof claim 6, wherein the layer of silicon oxide is chemically grown onthe silicon wafer.
 11. The method of claim 6, wherein growing thepassivated contact further comprises growing amorphous silicon on thelayer of silicon oxide.
 12. The method of claim 11, wherein growing thepassivated contact further comprises thermally crystalizing theamorphous silicon to form a layer of polycrystalline silicon (pcSi). 13.The method of claim 11, wherein the amorphous silicon is applied usingPlasma-Enhanced Chemical Vapor Deposition (PECVD).
 14. The method ofclaim 1, wherein removing the layer of the material comprises applyingat least one of an acid or a base to the layer of the material.
 15. Themethod of claim 1, further comprising applying a doped amorphous siliconcap on the passivated contact after removing the layer of the material.16. The method of claim 15, wherein the doped amorphous silicon cap hasa thickness between about 3 nm and about 7 nm.
 17. The method of claim1, wherein the passivated contact is incorporated within a solar cell.